Clones of Crazy Ant Queens Fuel Global Invasion

The worldwide invasion of Longhorn crazy ants appears to rely on a reproductive trick that allows for incest without the problems of inbreeding.

In the early stages of invasion, the average ant queen may have no choice but to mate with a male relative. But when that happens, her eggs can hatch as non-working males instead of workers.

Without workers, the colony quickly starves to death. So longhorn crazy ant queens avoid the problem by producing female offspring who are clones of themselves, and sons who are genetically unrelated clones of their fathers.

“It’s an incredibly bizarre system,” said study co-author Michael Goodisman, a Georgia Institute of Technology sociobiologist who described the trick Feb. 2 in Proceedings of the Royal Society B. “A queen produces males that are completely unrelated to her, that have none of her genetic material.”

Longhorn crazy ants, or Paratrechina longicornis, are so widespread that scientists don’t even know where they first came from. They form series of connected colonies, called “supercolonies,” that greatly disrupt ecosystems they invade, including human farms and homes.

To discover how crazy ant queens deal with a shortage of mates, Goodisman’s team formed 21 laboratory colonies, each with one queen and some workers. After three months, the researchers collected pupae of workers, males and queens, then analyzed their DNA.

Workers had one set of genes each from both mother and father, as normal. But females were exact copies of their queen mother, while males were clones of their fathers.

Because both the queens’ offspring are genetically unrelated, they can mate with one another without the consequences of inbreeding.

“It’s cheating, in a genetic sense, but this weird system allows [crazy ants] to overcome severe restrictions,” said evolutionary geneticist Jürgen Gadau of Arizona State University, who wasn’t involved in the study.

The cellular mechanisms of this phenomenon are unknown. Goodisman and Gadua suspect that female copies of genes are destroyed in eggs originally destined to be workers. But however it happens, it appears to be fantastically useful.

“It’s a way to skirt the deleterious effects of inbreeding and spread all over the world,” said entomologist Kenneth Ross of the University of Georgia, who was not involved in the study.